As shown in FIG. 5, a timing chain drive unit 500 in a dual overhead cam (DOHC) internal combustion engine, transmits power by means of a chain CH from a driving sprocket 550 to driven sprockets 560 and 570 which drive camshafts for operate intake and exhaust valves in a cylinder head. A pivoted slack-side chain guide 540, in sliding contact with a part of the chain CH that travels from the driving sprocket 550 to driven sprocket 560, cooperates with a tensioner T to apply appropriate tension to the chain. A pivoted lever of this type is described in Japanese Laid-open Patent Application No. 2003-214504.
A tension-side chain guide 510 is in sliding contact with a portion of the chain that travels from driven sprocket 570 to the driving sprocket to prevent vibration and lateral movement of the chain. Guide 510 controls the length of the span of the chain extending from the point at which the chain disengages the driven sprocket 570 to the point at which the chain engages the driving sprocket 550.
The slack side chain guide 540 is pivotally mounted for oscillating movement on a pivot shaft P, which can be a mounting bolt, a mounting pin, or the like, fixed to, and extending from, a wall of the engine E. The tensioner T biases a shoe on the pivoted slack side chain guide 540 against the chain. Whereas the slack side chain guide 540 is pivoted, the tension-side chain guide 510 is immovably fixed to the engine E by mounting bolts Q or other suitable mounting devices. This arrangement is shown and described in laid-open Japanese Patent Application No. 2003-214504.
In the conventional timing chain drive unit, the operation of the engine valves causes the torque load applied by the camshafts to their sprockets, and through the sprockets to the chain, to change cyclically in synchronization with the rotation of the camshafts and the crankshaft. These cyclic load changes result in corresponding cyclic changes in tension in the tension side of the chain, that is, the span of the chain traveling from the driven sprocket 570 toward the driving sprocket 550.
These cyclic changes in load and in timing chain tension are generated as a result of forces required to open the intake and exhaust valves. In the case of an in-line four cylinder engine, the change in tension goes through four cycles for each rotation of a camshaft. In an in-line six cylinder engine, the change in tension goes through six cycles for each rotation of a camshaft.
Cyclic changes in timing chain tension can also be caused by other factors such as cyclic changes in the rate of rotation of the crankshaft. The timing chain must have a tensile strength capable of withstanding the peak value of the varying chain tension. Accordingly, conventional timing chains are excessively heavy, the overall weight of the drive unit is high, and excessive noise is generated. Thus, the conventional timing chain drive unit is not well adapted to demands for size reduction, weight reduction and noise reduction in internal combustion engines.
Attempts to address the problems caused by cyclic variations in timing chain tension have included the use of non-circular sprockets, and sprockets having tooth gap bottoms located at varying radial distances from the sprocket axis. These approaches are described in United States patent publication 2007/0066430 and in U.S. Pat. No. 7,125,356. However, in the case of a non-circular sprocket or a sprocket having a varying tooth gap bottom radius, a force is applied to the chain in a direction perpendicular to its direction of travel, causing a corresponding displacement of the chain, and generating increased noise due to the vibration of the chain and impact between the chain and its chain guide.